THEORETICAL-STUDY ON THE REACTION-MECHANISM AND REGIOSELECTIVITY OF SILASTANNATION OF ACETYLENES WITH A PALLADIUM CATALYST

A theoretical study on the reaction mechanism of the silastannation of various alkynes with a palladium catalyst is presented. We investigat e the different regioselectivities in the reactions of SiH3SnH3 with m onosubstituted acetylenes (CN, H, CH3, and OCH3) catalyzed by Pd(PH3)( 2). The overall reaction scheme is first examined, and then the factor s for regioselectivities are analyzed. The rate-determining step is th e insertion of acetylene into the Pd-Sn or Pd-Si bond of the complex. Three factors are pointed out as governing the reactivity and regiosel ectivity. The first is the electronic factor which determines the rela tive stabilities of the transition states (TS) involving differently o riented acetylenes and those of the regioisomeric intermediates obtain ed after the TS. The overall reactivity is determined by the electron back-donation from Pd (home) to acetylene (lumo), while the stable ori entation of the substituted acetylene is determined by the electron do nation from the home of acetylene to the lumo (localized on Sn or Si) of the Pd complex. The second is the steric hindrance of the ligands. The steric repulsion of PPh(3) of a Pd(PPh(3))(4) catalyst is large en ough to give a different isomer from the one predicted by the electron ic factor. The third is the occurrence of the thermodynamic control wh en the products after the TS are unstable and therefore the reverse re actions can easily occur. The regioselectivities reported experimental ly and predicted theoretically here are reasonably explained by these three factors. The electron density distributions show electron donati on and back-donation, supporting the electronic mechanism proposed in this paper. An agostic interaction, between palladium and hydrogen, is found in the intermediate after the insertion step.